Typically, in a disk storage device used in a data processing system, data is stored on a rotating disk in a set of concentric circular patterns termed "tracks". A read/write head or transducer is mounted on a carriage incorporated in a servo system that moves the head radially in a "seek" or "access" function in which the servo system moves a read/write head to a selected track from a previous track. When the head reaches the desired track, the servo system commences a "track following" function in which it accurately positions the head over the centerline of the selected track and maintains it in that position so that the head can record a series of data bits or, alternatively, retrieve a series of bits from the track as the disk rotates under the head. Large-capacity disk drives may incorporate a plurality of such disks mounted for rotation together on a single spindle.
It is common to have the head-positioning servo system sense the position of the read/write head by means of servo signals recorded in tracks on the disk pack and correct for a mispositioning of the head. The servo signals may be embedded in the data, for example, recorded in servo fields at the beginnings of the data track sectors. Alternatively, the servo signals may be recorded on a disk surface dedicated to servo signals.
Particularly where a disk is removable from the drive, there is the possibility that the center of the circular tracks on which data was previously recorded is not coincident with the center of the spindle, due to disk slip or spindle imbalances. This causes an error or disturbance, called a "runout," that is periodic at the frequency of the rotation of the disk.
Prior art methods applicable to correcting the runout error have included the sampling of errors at various points along a data track, storing those errors, and later recalling and applying them to the servo controller. U.S. Pat. No. 4,136,365 to Chick et al. for "Magnetic Disc Storage Devices Having Compensation for Dimensional Changes," describes such a system. U.S. Pat. No. 4,536,809, to Sidman for "Adaptive Misposition Correcting Method and Apparatus for Magnetic Disk Servo System," describes another such system, which dynamically derives and stores optimal correction signals.
The prior art approaches, however, require significant memory and computer overhead for the calculation and storage of correction signals.